This invention relates to a method and apparatus for dry printing onto a workpiece having a contoured surface.
Dry printing is a decorating technique whereby a design is transferred from a flexible dry carrier, or "foil," onto a part that is to be decorated. The technique is widely used to apply decorative media to product surfaces such as instrument panels, name plates, dashboards, T.V., radio and other appliance cabinets, and so on. In its simple form, the foil (on which the design is carried prior to transfer) has four layers: a base web, which may be cellophane, paper, acetate, mylar or other film, usually in continuous strip form; a release layer such as wax or a resin, meltable when heat is applied to the uncoated side of the web; a decorative coating or print layer which is a combination of pigment and binder such as shellac; and a heat and pressure sensitive sizing or adhesive material formulated to adhere to the particular surface to be decorated.
In the "hot stamping" method of dry printing, the foil is positioned over the workpiece surface to be decorated and a heated die presses an area of the foil against the surface. Heat transfers from the die to the foil, causing the release layer to melt, thereby releasing the decorative layer from the base web, and activating the adhesive on the opposite side of the print layer so that the print layer adheres to the article when the web is removed.
The invention relates to the so-called "roll on" type of dry printing process, wherein the foil is engaged with the workpiece by a roller, as the workpiece is passed beneath the roll, rather than stamped by a die. A further background description of dry printing techniques is given in Appliance Manufacturer, August 1973.
In each method, the release of the printing layer from the foil and its transfer and adhesion to the workpiece requires the application of pre-established conditions of heat, pressure and dwell time.
The hot stamping method can be used to apply designs to angulated surfaces, and to surfaces which are curved or contoured, by use of a die which is contoured to match the surface to be decorated. That technique requires, of course, a separate die for each particular type of contoured surface to be decorated. Moreover, it requires some accuracy in the alignment of the workpiece with respect to the die, so that the die will properly engage the workpiece. The hot stamping method suffers from the defect that occasionally bubbles of air are trapped between the design layer and the workpiece surface, especially where a design is being applied over a large area at one time.
The roll-on method does not generally require specially shaped rolls, and it substantially obviates the difficulty of bubble entrapment. However, roll-on techniques have been limited to use with surfaces which are essentially horizontally oriented, i.e., uncontoured (in the direction of workpiece movement relative to the roll). Previous roll-on presses have not been able to provide or maintain the needed constancy of heat, pressure and dwell time on contoured surfaces. Workpiece surfaces having a substantial contour have, therefore, required hot stamping.
Previous roll-on type dry printing machines have included a conveyor which advances the part to be printed beneath the roller, which is mounted above the conveyor in a carriage. The roll is heated by radiation from heating elements in a semicircular hood above the roll. The foil is fed beneath the roll, rotation of the roll conveying heat from the heating element to the foil. The carriage which carries the roll is adjustable vertically, to enable the roll height to be set to match the size of a given workpiece. The roll is biased downwardly, toward the workpiece, by spring pressure, but the useful range of vertical travel has been limited to a fraction of an inch because bias spring forces change to rapidly over greater distances and therefore do not maintain the application pressure within an acceptable range. The limited roller travel has thus been sufficient to accommodate only minor contouring such as the surface irregularities normal to manufacturing processes.
The foregoing limitations of the roll travel range, within which there is acceptable constancy of roll-on force, have prevented use of roll-on processes for applying dry print designs onto surfaces having a high degree of contouring, such as around a curved corner from one surface onto an adjacent surface. Thus, it has not been possible, in a single pass, to decorate, for example, an automatic grille having two surfaces which meet at an angle at the center. In the past such contoured surfaces have had to be decorated in sequential steps by a hot stamping process with a die corresponding to the contour of each distinct surface area, or by mounting the part so that each surface is presented horizontally for roll-on decoration.